Such a long electrical motor is known from U.S. Pat. No. 2,251,816, where the bearings are arranged in a groove in the rotor and consequently are made up of bearing rings that might be split in two half bearing rings held in place by butterfly keys. Such rings might contain a roller bearing in an internal groove for reducing the friction. The production and mounting of such bearings is elaborate and time consuming and therefor expensive. Further, the heat transfer properties are dependent on the stator, as the bearing rings are supported directly by the stator.
Another long electrical motor type is known from U.S. Pat. No. 3,136,905 and from DE-B 885 279, where the stator is divided in sections by bearing parts being much thicker than the single stator disc, but where the windings of the stator are cut off by the bearing parts and are not passing from one end to the other of the long electrical motor. Further, in U.S. Pat. No. 3,136,905 the bearings are not of a "low friction" type. Such motors is also very complicated to assemble and has a low degree of reliability due to the complicated production of the stator windings. This construction, also, ends up with a bad efficiency, as the magnetic flux is scattered by the relatively large gaps in the stator windings.
Further, a long electrical motor is known from U.S. Pat. No. 3,136,905, where the bearings are defined directly by the stacked discs between the sections. Those discs are thus made from a material that is not well suited for the purpose of defining a bearing with an uncontrolled friction. A bearing made of what could be regarded as a laminated material will under a relatively large friction provide a non-uniform wearing surface, thereby making a pattern of abrasion in the rotating axis. The friction causing an increase in the temperature is dependent on good heat transfer properties of the bearings. Such good heat transfer properties are not present, when the bearings are made from the stacked discs, as iron (Fe) has thermal conductivity of only 72.8 J/(m.s.K).
From U.S. Pat. No. 5,128,573 another long electrical motor with multiple sections is known, where a rotor bearing and sleeve assembly is arranged between each rotor section, each bearing including a fixed key for positioning in longitudinal spacings formed as part of the stator section, to prevent rotation of the bearing during use. Such bearing is evidently not a low friction type, as the fixed key evidently is necessary.
From U.S. Pat. No. 4,638,198 still another long electrical motor with multiple sections is known, where the upper rotor section of each pair of successive rotor sections may have its lower end supported on a rotor shaft protrusion, such as a ring mounted in an annular groove of the rotor shaft, and where further a chamfer at the end of the rotor bore locks the ring in place. Although this prior art has a protrusion for the support of the lower end, this protrusion can not be characterized as a bearing of low friction type. Also, the protrusion is locked on place by a chamfer at the end of the rotor bore, which is not reducing the friction.
Finally, a long electrical motor with multiple sections in known from U.S. Pat. No. 4,453,099, where the bearing is an assembly including a sleeve, that is keyed to the shaft, an inner portion, that rotably receives the sleeve, and a flange with a periphery, that frictionally engages the stator to prevent the bearing from rotating.
The state of the art has some drawbacks in that the threadening of the stator windings all the way through the stator package is not possible or very difficult. The threadening is very time consuming and expensive, and a means for reducing the costs, when threadening the stator windings in a long electrical motor, would be most convenient. Further, the state of the art is also dependent on the friction in that the friction in the bearings is not controlled, and the efficiency is relatively low due to the large magnetic gap between rotor and stator, necessary to avoid mechanical damages when the magnetic momentum is large. Finally, the costs for production and maintenance are high. The friction can lead to heating and premature wear of the bearings and will therefore cause loss of energy and materials.